The present invention relates to two-piece molded products consisting of a cover comprising an ionic copolymer and a center part containing cis-polybutadiene or other elastomers, and a monomer capable of cross-linking the elastomer into a three-dimensional network. More specifically, the cross-links in the center part are relatively long and flexible. Such molding compositions are eminently suitable for the production of molded golf balls, particularly unitary molded golf balls. Molding compositions of this general class, and unitary molded golf balls which can be produced from them, are described in U.S. Pat. No. 3,313,545 issued Apr. 11, 1967 and U.S. Pat. No. 3,438,933 issued Apr. 15, 1969.
There are several advantages of homogeneous, unitary construction for a golf ball, in contrast to the wound golf balls of the earlier art. Unitary golf balls can be produced with a perfect center of gravity and thus have excellent aero-dynamic properties, superior roll, and trueness of flight. Such golf balls are highly resistant to cutting, and are often indestructible in normal play. These balls will return to round even when severely distorted, and thus maintain their superior flight characteristics after extended use. Homogeneous, unitary golf balls may be manufactured with better quality than conventional wound balls.
As contrasted with the conventionally covered wound balls, unitary balls maintain their playing characteristics in hot and cold weather, have an excellent shelf-life and will not water-log. If the paint on the golf ball becomes worn or damaged, the balls may be very readily reclaimed by removing or stripping off the old paint and repainting. By contrast, conventional wound balls seldom last long enough to allow repainting.
Such unitary balls may be molded in mating precision hemisphere molds or dies into which a cylindrical or other shaped slug of moldable material is placed, as described in U.S. Pat. Nos. 3,313,545 and 3,438,933, the disclosures of which are incorported herein by reference. The slugs may be cut from the extrudate of a mixer-extruder. After placing of the slug, of sufficient size to fill the mold, the mold halves are closed, and heat and pressure are provided for enough time to cure the moldable elastomer. The moldable material comprises an elastomer, a cross-linking monomer, a filler and a cross-linking catalyst. The balls after discharge from the mold are buffed, surface treated and then painted and stamped in conventional manner.
Improved unitary molded golf balls are described in U.S. Pat. No. 4,056,269, issued Nov. 1, 1977. The improved unitary golf balls utilize a specific sub-group of cross-linking monomers and can yield molded golf balls with better rebound than prior unitary golf balls, superior playing qualities equal to or surpassing the highest quality wound golf balls available and maintaining the advantages of unitary golf balls. The improved unitary balls were made with decreased amounts of filler, preferably less than about 30 phr (parts per 100 parts by weight of resin) of filler and more preferably less than about 15 phr of filler. These low filler additions are postulated to give higher rebound and superior distance to the improved golf balls. However, golf balls made commercially by the normal procedures described in U.S. Pat. No. 4,056,269 have several deficiencies. To produce a golf ball having a suitable compression and one which is commercially acceptable to the normal golfer, it is necessary to use substantial amounts of monomer, preferably over 30 parts. These golf balls with higher monomer content do not possess flight distance equivalent to the best wound balls and, in fact, are several yards shorter than these high quality golf balls although still superior to the unitary golf balls described in the original patents.
Another shortcoming of the improved golf ball is a tendency of the golf ball to chip in prolonged use although the golf balls remain highly resistant to cutting. This tendency towards chipping is attributed to the highly exothermic reaction occurring in the center of the golf ball and resultant high temperatures noted in the center, over 220.degree. C., as contrasted with the lower temperature of the press at the skin of the golf ball. The wide gradient in temperature across the width of the golf ball is postulated to encourage poor chip-resistance in these improved golf balls.
Another deficiency in the improved golf ball is the lower temperature and consequently longer cure cycle needed to obtain good durability. The original solid molded golf balls are readily cured in short cycles of about 10-20 minutes at high temperatures of 170.degree. to 180.degree. C. When these same cycles are applied to the improved golf ball using the metal-containing monomer, marked reductions in durability are noted, as evidenced by a decrease in cannon life as the cure temperature is raised to 170.degree. to 180.degree. C. Accordingly, the improved golf balls are made at 150.degree. to 160.degree. C. and cure cycles of 30 to 35 minutes.
Another deficiency of the improved golf ball is the existence of unduly heavy tough flash at the equator of the balls which causes increased work to obtain clean looking surfaces and also a number of rejects due to the thickness and unevenness of the flash.
While such improved golf balls are quite satisfactory and do constitute an improvement over the standard unitary golf balls with respect to distance, rebound and click, it is an object of this invention to improve further the properties of such molded golf balls with respect to flight distance, flight pattern, rebound, durability, and ease of manufacturing as reflected by shorter cure cycles, elimination of rejects due to faulty buffing and stamping.